ip dhcp snooping - 3750 can't contact db srvr & snooping not wrkng

Unanswered Question
Feb 9th, 2009

I have a 3750 that is acting as a dhcp srvr for vlan 2. I have 5 other 3750s that connect to this switch. All 5 are configured identically and have the same IOS. However, ip dhcp snoop is not working on one. Attached is a document with the sh commands and the run config from this non-working switch. The tftp server can be reached. I can manage the switch. If I connect a Ethernet switch to the 3750 and plug a laptop into one of its ports, I cannot get a vlan 2 ip. If I put that port in vlan 1, I get an ip (from the vlan 1 dhcp srvr). What am I missing? I did a file compare between a working switch and the non-working switch and as I said there is nothing missing.

I have this problem too.
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Yudong Wu Mon, 02/09/2009 - 10:48

Can you connect your laptop to the problem 3750 directly and configure the port in access mode of vlan 2? If your laptop can get IP in vlan 2 in this way, the issue should be between this 3750 and the testing switch? By the way, what kind of testing switch are you using? Did you check if the trunk status between them? Are vlan 2 is allowed on the trunk?

murray-davis Mon, 02/09/2009 - 12:05

I was just to about to update my post. Kwu2, you are spot on. The problem was that the 3750 did not have a vlan 2 database. "J'accuse Cisco!" When I issued the command: ip dhcp snooping vlan 2, I was returned to the command prompt. No error message! After I issued that command and then issued: sh ip dhcp snooping, the output showed that snooping was configured on "none" for vlans. I then just took a random port and configured it for vlan 2. At that point, the switch configured the vlan 2 database and ip snooping started to work.

I would have thought that the switch would have at least warned me that vlan 2 was not in the vlan database. Boohoo!


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